[ISSUE #8909] Move nullcheck ahead (#8910)

Co-authored-by: jiaoja <[email protected]>

Author: jjastan

common/src/main/java/org/apache/rocketmq/common/config/AbstractRocksDBStorage.java

@@ -346,12 +346,13 @@ protected void open(final List<ColumnFamilyDescriptor> cfDescriptors) throws Roc
             this.db = RocksDB.open(this.options, this.dbPath, cfDescriptors, cfHandles);
         }
         assert cfDescriptors.size() == cfHandles.size();
-        try (Env env = this.db.getEnv()) {
-            env.setBackgroundThreads(8, Priority.LOW);
-        }
+
         if (this.db == null) {
             throw new RocksDBException("open rocksdb null");
         }
+        try (Env env = this.db.getEnv()) {
+            env.setBackgroundThreads(8, Priority.LOW);
+        }
     }
 
     protected abstract boolean postLoad();